The invention relates generally to vacuum insulation panels. More particularly, the invention relates to an improvement for eliminating deformation in a fastening surface for a vacuum panel.
Vacuum insulation panels are now in widespread use in thermal insulation applications which include refrigerators, ovens and cryogenic storage vessels. These panels are extremely efficient at insulating against heat transmission. The insulating efficiency of the panels depend on the degree of evacuation achieved on the panel interior during manufacture and on the ability of the panel to maintain the internal vacuum throughout its useful life.
Referring to FIGS. 1 and 2, known vacuum panels are typically formed by welding a pan-shaped base 10 to a flat cover 12 to define an internal space 25. FIG. 2 illustrates a cross-section of a known vacuum panel, wherein dotted lines denote the panel shape prior to application of a vacuum to the internal space 25 and solid lines denote the panel shape after application of the internal vacuum. As used herein the term "evacuation"0 refers to the application of an internal vacuum to the panel. The term "atmospheric loading"0 refers to the forces due to atmospheric pressure on a panel which has been evacuated. Base 10 is formed as a generally rectangular bottom surface 15 which extends into a smoothly rounded peripheral corner section 13. Corner section 13 further extends into an inclined wall 17, and an outer flange 14. Cover 12 is formed as a planar sheet and is joined to flange 14 of the base 10 by welding to hermetically seal the panel. Cover 12 forms a fastening surface 30 by which the vacuum panel is bonded to a generally planar target surface (represented by the line 40), which is the surface desired to be mounted and insulated by the panel.
Application of the internal vacuum is accomplished via one or more seal-off ports (not shown) provided on the base or cover. The seal-off port is typically provided with slot apertures disposed in a recessed portion of the panel. Solid braze material is placed in the recess adjacent the apertures so that molten braze material flows into the slots to seal the panel. The entire panel is usually placed in a vacuum chamber where the panel is sealed after appropriate steps have been taken to ensure evacuation of the internal space. Sealing of the seal-off port occurs within the vacuum chamber using a specialized radiant heater head designed for local heating of the braze.
The base 10 and cover 12 are preferably formed from sheet metal, which provides an excellent barrier to gases that would migrate into the interior vacuum cavity during the life of the panel. Preferably, both the cover and base are made of 3 mil stainless steel; however, carbon steel or other suitable material may be used. For example, T304L stainless steel is particularly well suited for the vacuum panel of the invention because it is not gas permeable and is cost effective, readily available, formable, has low outgassing, good corrosion resistance, low thermal conductivity, and a high melting temperature.
A fiberglass mat 20 is typically placed within the panel before the base 10 and cover 12 are welded together. The glass mat is compressed during assembly of the panel and provides added insulation characteristics as well as structural support for the panel walls. Mat 20 may comprise, for example, a dense glass wool manufactured by Owens-Corning Fiberglass, Toledo, Ohio having a density in the range of 9.0 to 20.0 pounds per cubic foot. The mat 20 is compressed during assembly of the panel and functions to provide insulation against heat transfer and to support the panel walls against the forces of atmospheric pressure.
Generally, a lower density glass mat has a higher effectiveness as an insulator. On the other hand, a lower density mat provides less structural support to the panel walls. Thus, some insulating efficiency may be sacrificed in order to achieve a given degree of structural support for the panel walls.
Referring again to FIG. 2, one problem with prior art vacuum panels is that they tend to deform under atmospheric loading. Fastening surface 30 develops steps 35 under atmospheric loading as previously flat cover 12 collapses inwards toward the center of the panel. The panel is fastened to the target surface by applying adhesive to the fastening surface 30.
Atmospheric loading of the evacuated panel also yields deformation in the fastening surface 30 by causing it to buckle. Buckling results from a reduction in the lateral dimension of the base 10 as walls 17 and corners 13 move inwards under atmospheric forces. Bottom surface 15 also buckles upwards since the shrinkage in the lateral dimensions of the panel base 15 cannot be accomodated by bottom surface 15.
As can be seen from FIGS. 2 and 4A, the deformed fastening surface 30 of prior art panels results in gaps 42 between the fastening surface 30 and the target surface 40. FIG. 4A is exagerrated somewhat to illustrate the shape undertaken by the target surface 40 when the prior art panel is bonded thereto. Compression resulting from evacuation of the panel causes flange 14, and that part of cover 12 attached to flange 14, to be raised with respect to fastening surface 30. The raised flange 14 contributes to a reduction in the adhesion surface area available and to deformation of the surface to which the panel is bonded. The target surface 40 is typically thin sheet metal (0.035 in.) and conforms to the shape of fastening surface 30. This conformance of the target surface creates large depressions therein which are cosmetically undesirable. These imperfections decrease the effectiveness of the vacuum panel as a thermal insulator and reduce the surface area available to fasten the panel to the target surface.
Another problem with prior art panels is that, during assembly, the fiber glass mat material may migrate between the cover and base flanges, contaminating the welds therebetween. Since the glass mat is necessary to provide support against atmospheric loading to the cover, even in the area of the flange welds, the problem of weld contamination has heretofore been tolerated as a necessary consequence of the panel design.
Another problem with prior art panels is that during assembly, the fiber glass mat material must be compressed with a load equal to atmospheric pressure forces. This is necessary to reduce the amount of warping of the fastening surface 30 and is undesirable during the manufacturing process because very large forces are frequently required for panels having large surface areas.
There is thus desired a vacuum panel which provides a fastening surface that does not tend to deform as a result of atmospheric loading and which facilitates complete contact with the target surface. There is also desired a vacuum panel which provides increased weld quality between the cover and base members and which solves the aforementioned problems of the prior art.